The present invention relates to electromagnetic stimulation of target areas, typically within the anatomy of a living organism such as a human medical patient or an animal. More specifically, the invention relates techniques for utilizing time-dependent and space-dependant variables to focus electromagnetic energy on a target area.
Transcranial Magnetic Stimulation (TMS) and Repetitive Transcranial Magnetic Stimulation (rTMS, a variant of TMS in which electromagnetic fields are produced in trains of multiple short pulses) have shown the ability to trigger neuronal firing in selected superficial brain regions. In at least one psychiatric condition (major depression), this effect of TMS and of rTMS appears to constitute an effective therapy. TMS and rTMS instrumentation are currently limited by their inability to focus their magnetic fields at depth. This is chiefly because a magnetic field always diminishes as a function its distance from the source.
Attempts have been made to focus electromagnetic energy into deep structures without overwhelming superficial structures. For example, it has been suggested to simultaneously use multiple coils such that the magnetic fields converge at a chosen point (see Sackheim, H A. Magnetic Stimulation Therapy and ECT (Commentary) Convulsive Therapy, 1994, 10(4): 255-8). Even if feasible, the coordination of multiple coils (e.g., adjusting for a specific target) may make the results less than satisfactory.
In U.S. Pat. No. 6,572,528, the inventors propose the use of an adaptation of a 1.5 Tesla MRI scanner to produce some form of transcranial magnetic stimulation. Because the largest of the magnets on such a machine (the solenoid) remains stationary and at steady state while the programmable magnets (e.g., the head coil and the gradient coil) are of relatively low field strength, such a configuration may not be capable of targeting for performing selective stimulation of targeted deep brain structures while sparing superficial structures.
A variety of new electromagnet configurations have been developed by the Helsinki group (Ruohonen, J, Dissertation for Doctorate of Technology, Helsinki University of Technology, Espoo, Finland, 1998), which may be useful in the context of TMS for reaching to deeper structures. However, these static magnets pass the greatest portion of their energies through interposed proximal tissue, and hence cannot alone achieve deep TMS while sparing proximal tissue.
In the article “A Coil Design for Transcranial Magnetic Stimulation of Deep Brain Regions” (Roth, Y; Zangen, A; Hallet, M; Journal of Clinical Neurophysiology, 2002, 19(4):361-370), the authors describe the “Hesed” coil shape, which reportedly has a less sharp drop-off in power with distance from the coil. Additionally, a configuration of multiple coils has been attempted to stimulate deep brain structures. (George, M S Stimulating the Brain, Scientific American, editor's inset window, page 72 September 2003). As mentioned above, even if such an approach does prove to be feasible, it is also likely to be expensive and inflexible. For example, targeting different brain regions may require a different coil array and even targeting the same structure in two different individuals may require two different sets of hardware.
Both mechanical and computerized stereotactic neurosurgical image guidance systems such as the STEALTH STATION by Surgical Navigation Technologies, Inc., Broomfield Colo. (Division of Medtronic Inc.) have been fitted to TMS coils, in an attempt to better aim the magnetic field at the targeted structure. However these approaches have met with limited success because of the principle of electromagnetism that the electromagnetic field is always greatest next to the surface of the coil than it is at any given distance away from that coil. Hence, even when carefully aimed with expensive image guidance equipment, superficial neuronal structures continue to be overwhelmed before targeted deep structures can be stimulated.
There exist devices designed to distribute other forms of energy lightly to the proximal periphery, while concentrating it at a distal target point. U.S. Pat. No. 5,207,223 (Adler, J R, 1993) describes a method for manipulating a radiation beam source so that emitted radiation affects a target internal to the human body while minimizing peripheral radiation damage. U.S. Pat. No. 5,427,097 (Depp, J G 1995) provides further methodology for this purpose and the CyberKnife® device (Accuray, Inc., Sunnyvale, Calif.) is a radiosurgery robot that functions using the technologies described in those patents.
Magnetic fields differ from radiation beams in the manner that they emanate from their sources, their physical parameters, the methods by their parameters may be altered, and the manner in which they affect living tissue to achieve a desired effect. Consequently, satisfactory techniques by which magnetic field sources can be moved and otherwise manipulated in real time so as to selectively affect deep targeted structures while leaving superficial structures relatively undisturbed and avoiding undesirable side effects such as seizures have not been developed. Additionally, it would beneficial to provide repetitive transcranial magnetic stimulation that can selectively stimulate deep brain tissue without overwhelming superficial cortical brain structures. Further, robotically manipulating TMS sources and automatically altering their magnetic field parameters as a function of the instantaneous coil location relative to the designated target would be desirable. Finally, it would be beneficial to have transcranial magnetic stimulators that are able to stimulate or suppress arbitrarily selected neuronal areas by changing numerically or graphically selected target coordinates.